fix: keep ACP stdout protocol-clean

Route AIAgent print output to stderr via _print_fn for ACP stdio sessions.
Gate quiet-mode spinner startup on _should_start_quiet_spinner() so JSON-RPC
on stdout isn't corrupted. Child agents inherit the redirect.

Co-authored-by: Git-on-my-level <Git-on-my-level@users.noreply.github.com>

tools/delegate_tool.py

@@ -197,6 +197,18 @@ def _build_child_agent(
     # total iterations across parent + subagents can exceed the parent's
     # max_iterations.  The user controls the per-subagent cap in config.yaml.
 
+    child_thinking_cb = None
+    if child_progress_cb:
+        def _child_thinking(text: str) -> None:
+            if not text:
+                return
+            try:
+                child_progress_cb("_thinking", text)
+            except Exception as e:
+                logger.debug("Child thinking callback relay failed: %s", e)
+
+        child_thinking_cb = _child_thinking
+
     # Resolve effective credentials: config override > parent inherit
     effective_model = model or parent_agent.model
     effective_provider = override_provider or getattr(parent_agent, "provider", None)
@@ -226,6 +238,7 @@ def _build_child_agent(
         skip_context_files=True,
         skip_memory=True,
         clarify_callback=None,
+        thinking_callback=child_thinking_cb,
         session_db=getattr(parent_agent, '_session_db', None),
         providers_allowed=parent_agent.providers_allowed,
         providers_ignored=parent_agent.providers_ignored,
@@ -234,6 +247,7 @@ def _build_child_agent(
         tool_progress_callback=child_progress_cb,
         iteration_budget=None,  # fresh budget per subagent
     )
+    child._print_fn = getattr(parent_agent, '_print_fn', None)
     # Set delegation depth so children can't spawn grandchildren
     child._delegate_depth = getattr(parent_agent, '_delegate_depth', 0) + 1